Electrodeposition of rust-preventive primers on metal automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of an electrodepositable coating composition comprising an aqueous emulsion of a film forming polymer and the article acts as an electrode in the electrodeposition process. A high voltage electric current is then passed between the article and a counter-electrode in electrical contact with the coating composition until a coating of a desired thickness is deposited on the article. In a typical cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
After the electrodeposition process is complete, the resulting coated article is removed from the bath and is rinsed with deionized water and then cured typically in an oven at sufficient temperature to form a crosslinked finish on the article. Once the electrodeposition rust-preventive primer is applied to the automotive substrate, the vehicle is then top coated with a multi-layer automotive exterior finish to provide chip resistance properties and an attractive aesthetic appearance such as gloss and distinctness of image.
One disadvantage associated with conventional electrodeposition processes is that coating defects tend to form on the surface of the coated article, such as pinholes and cracks, which can compromise the corrosion protective properties of the electrodeposited film and produce other deleterious effects such as a rough film surface. The high voltage baths required in electrodeposition coating processes use up large amounts of electricity and are also expensive to maintain. Furthermore, the multiple deionized water rinses are undesirable, as they present significant waste handling and water treatment problems.
Accordingly, there is a desire to eliminate the electrocoating process altogether and find new coating methods and compositions which can replace the electrodeposition process, while still maintaining the desired coating properties for automotive rust-preventive primer finishes such as a high degree of corrosion resistance and paint adhesion to both underlying rust-preventive pretreatments on the metal surface and to paint applied thereover during exterior automotive finishing operations.
Various ionomeric coating compositions comprising aqueous dispersions of ionomer resins made from ion-neutralized ethylene-acrylic acid or ethylene-methacrylic acid copolymers have been proposed for rust-preventive treatment of metal surfaces, for example, as disclosed in JP 2000-198949 A2 to Akimoto et al., WO 00/50473 A1 to Nakata, et al., and U.S. Pat. No. 6,458,897 to Tokita, et al. issued Oct. 1, 2002.
However, diverse properties are required for a coating formed from an ionomer resin dispersion in order for it to be a suitable commercial replacement for an electrocoat bath. When conventional ionomer aqueous dispersions are tried, the resulting coating cannot provide the level of bath stability and uniformity and corrosion resistance, water impermeability, film smoothness and ease of use required to produce a high performance rust-preventive coating of automotive quality.
The present invention provides a uniform aqueous dispersion of an ionomer resin with good stability and rust-preventive properties that sufficiently answers the high performance requirements of automotive finishes and therefore is suitable as a commercial replacement for conventional electrodeposition primers used nowadays in automotive assembly plants. The present invention can also be applied directly over unplated metal to provide direct contact corrosion protection, which provides substantial savings to the automakers, since most vehicle bodies today are constructed of costly Zn plated (galvanized) steel everywhere except for the roof area.